Abstract: An acid agent comprises a liquid water-insoluble product of concurrent reaction of:(a) one mole of an hydroxy(R)-methyl derivative of phenol in which 0.4-2.0 moles of HO--CH(R)-groups are present per one mole of phenol, wherein R equals H--, CH.sub.3 --, or(b) one mole of phenol and 0.3-2.0 moles of an aldehyde and(c) 0.03-100.0 moles of an aryl sulphonic acid, having an acid number from 10 to 300 (mg.KOH/g) with a maximum water content of 35% by weight.The process of reacting said components is conducted in a flow reactor for 0.1-10 minutes at 50.degree.-150.degree. C.The acid agent according to the invention may be modified with other substances, such as amines, alcohols, or acids to impart additional useful properties thereto. The acid agent has low viscosity, low corrosive activity, improves the strength of phenol-formaldehyde resins cured therewith by 30% and lowers their water absorption capacity by 2-10 times.

Abstract: A process for producing an acid agent which comprises reaction of sulphonated phenol and formaldehyde in an "ideal" displacement reactor at a temperature within the range of from 50.degree. to 120.degree. C. at a constant molar ratio of said components. The reaction may be carried out in the presence of additives such as urea or orthophosphoric acid.The process according to the present invention has an increased productivity at reduced power and labor inputs as compared to the prior art processes.